The invention relates to a method of and an arrangement for testing ion concentrations in solutions using ion sensitive test and reference electrodes.
In many cases electrochemical processes are used in order to determine ion concentrations in a solution. Potential measurement with ion selective electrodes is a testing technique which is easy to manipulate and adapted to modern electronics. Ion-selective electrodes are electrochemical half cells in which there is a potential difference at the phase boundary between the electrode material and the electrolyte. This potential difference .DELTA..psi. is related to the concentration (more precisely activity) of the type of ions to which the electrode is to be sensitive. In theory this relationship is substantiated by Nernst's equation: EQU .DELTA..psi.=.DELTA..psi..sub.o +(RT/zF)ln a.sub.Me.spsb.z+ (1),
where T is the absolute temperature, R is the molar gas constant, F is the Faraday constant and z is the valency of the particular type of ions Me.sup.z+. .DELTA..psi..sub.o is the potential of the cell half for the ion activity a.sub.me.sup.z+ =1. The potential .DELTA..psi..sub.o is called the standard potential. A plurality of ion sensitive electrodes are known. Not only is it possible to determine simple inorganic ions but also those of amino acids and complex organic compounds such as enzymes and proteins, for example.
Potential measurements in electrochemistry are carried out by measuring the potential difference between a test electrode and a reference electrode. In addition the test electrode should respond as selectively as possible to the ion which is to be investigated. On the other hand the reference electrode should not be sensitive to impurities in the test solution (electrolyte). In some circumstances the two electrodes have to be dipped into separate electrolytes, a test electrolyte and a reference electrolyte (standard solution), the latter being connected by means of a so-called current key. A current "key" consists of a curved glass tube or capillary which connects the two electrolytes and contains a salt solution the cations and anions of which have the same mobility. Such an arrangement is very expensive.
It has been proved by experience in electrochemistry that the values for the absolute potentials at these electrodes, more particularly at ion-sensitive electrodes, exhibit disruptive fluctuations caused, for example, by undesirable chemical changes in the electrodes. Therefore it is usual to calibrate with the aid of a standard solution each time before using an electrode. However the relative dependence of the potential on the activity of the ion which is to be measured doesn't change significantly in time. In a number of electrodes the value predetermined by Nernst's equation is even achieved. Therefore one calibration point only is frequently determined for series tests, the one calibration point fixing the absolute amount of the potential, whereas the manufacturers' specifications are relied upon for the dependence of the potential on concentration or a dependence in accordance with Nernst's equation is stipulated. The properties of both electrodes are included in measurement of potential as well as the properties of the test and reference electrodes. In the case of reference electrodes, fluctations in the (reference) potential of approximately 5% can occur.
A so-called ion-sensitive field effect transistor (ISFET) can be used as the test electrode and is described for example in the journal Ion-Selective Electrode Review, Vol. 1, 1979, pages 31 to 79, J. Janata and R. J. Huber "Ion-sensitive Field Effect Transistors".
These ion-sensitive field effect transistors (ISFETs) also make it possible to transform the ion concentrations of a solution into an electrical signal. In addition, a potential difference is formed between the solution and the ion-sensitive gate of the ISFETs. However, this potential difference is not measured directly; instead the drain/source current of the ISFETs which is affected thereby is measured. The drain/source current of the ISFET is therefore a measure of the electrical potential at the gate electrode which is in direct contact with the electrolyte to be measured. However, a reference electrode which determines the gate potential is necessary in this case, too, as a reference and in order to fix the operating point. Ultimately therefore, the accuracy and reproducibility of the reference electrode is decisive in the use of ISFETs. Therefore, there is no basic progress to be made over the classical method of potential measurement with two electrodes, with this particular use of an ISFET as a test electrode.